The invention concerns a device for coating a web of material the coating being applied either directly to the web as it travels around the backing roller or indirectly, to the surface of a roller and thence to the web.
Nozzle-based devices are preferred for applying low-density (&lt;10 g/m.sup.2) coatings to papers traveling at high speeds (&gt;700 m/min). Essential components of known generic nozzle-based coaters are
a coating chamber that opens toward the backing roller and is supplied with liquid coating from a preliminary-distribution chamber through a gap that extends over the operating width and PA0 a flow-control system (usually involving a doctor blade) closing off the downstream end of the coating chamber. PA0 that extends over the operating width at the outlet end of the coating chamber and terminates just upstream of the flow-control system, PA0 that is demarcated at one end by the backing roller and at the other by a baffle that essentially parallels the surface of the backing roller, PA0 that extends along the direction of travel at least about 50% of the distances between the overflow plate and the flow-control system, PA0 that is open along its width perpendicular to the axis of the backing roller from about 1.5 to 15%, and preferably from about 2.5 to 5% of its length, and PA0 that terminates just upstream of the flow-control system.
The intake end of the coating chamber is demarcated by an overflow plate that terminates in the vicinity of the backing roller, leaving an overflow gap. The coating as it flows through the overflow gap prevents air from entering the coating chamber along with the web. The chamber is sealed off laterally at the ends of the backing roller by boards.
Coating devices of this type are described in German GM 8 424 904 and German Patent 3 336 553.
One drawback of the known nozzle-based coaters is that the quality of the coating is very poor at high speeds (&gt;800 m/min). The higher the speed, the more serious the problems with spots, streaks, bubbles, etc.